The present invention relates generally to memory storage devices. More particularly, the invention is a method and apparatus for interfacing an external computer to a set of storage devices which are typically disk drives.
Magnetic disk drive memories for use with digital computer systems are known. Although many types of disk drives are known, the present invention will be described as using hard disk drives. However, nothing herein should be taken to limit the invention to that particular embodiment.
Many computer systems use a plurality of disk drive memories to store data. A common known architecture for such systems is shown in FIG. 1. Therein, computer 10 is coupled by means of bus 15 to disk array 20. Disk array 20 is comprised of large buffer 22, bus 24, and a plurality of disk drives 30. The disk drives 30 can be operated in various logical configurations. When a group of drives is operated collectively as a logical device, data stored during a write operation can be spread across one or more members of an array. Disk controllers 35 are connected to buffer 22 by bus 24. Each controller 35 is assigned a particular disk drive 30.
Each disk drive within disk drive array 20 is accessed and the data thereon retrieved individually. The disk controller 35 associated with each disk drive 30 controls the input/output operations for the-particular disk drive to which it is coupled. Data placed in buffer 22 is available for transmission to computer 10 over bus 15. When the computer transmits data to be written on the disks, controllers 35 receive the data for the individual disk drives from bus 24. In this type of system, disk operations are asynchronous in relationship to each other.
In the case where one of the controllers experiences a failure, the computer must take action to isolate the failed controller and to switch the memory devices formerly under the failed controller's control to a properly functioning other controller. The switching requires the computer to perform a number of operations. First, it must isolate the failed controller. This means that all data flow directed to the failed controller must be redirected to a working controller.
In the system described above, it is necessary for the computer to be involved with rerouting data away from a failed controller. The necessary operations performed by the computer in completing rerouting requires the computer's attention. This places added functions on the computer which may delay other functions which the computer is working on. As a result, the entire system is slowed down.
Another problem associated with disk operations, in particular writing and reading, is an associated probability of error. Procedures and apparatus have been developed which can detect and, in some cases, correct the errors which occur during the reading and writing of the disks. With relation to a generic disk drive, the disk is divided into a plurality of sectors, each sector having the same, predetermined size. Each sector has a particular header field, which gives the sector a unique address, a header field code which allows for the detection of errors in the header field, a data field of variable length and ECC ("Error Correction Code") codes, which allow for the detection and correction of errors in the data.
When a disk is written to, the disk controller reads the header field and the header field code. If the sector is the desired sector and no header field error is detected, the new data is written into the data field and the new data ECC is written into the ECC field.
Read operations are similar in that initially both the header field and header field error code are read. If no header field errors exist, the data and the data correction codes are read. If no error is detected the data is transmitted to the computer. If errors are detected, the error correction circuitry located within the disk controller tries to correct the error. If this is possible, the corrected data is transmitted. Otherwise, the disk drive's controller signals to the computer or master disk controller that an uncorrectable error has been detected.
In FIG. 2 a known disk drive system which has an associated error correction circuit, external to the individual disk controllers, is shown. This system uses a Reed-Solomon error detection code both to detect and correct errors. Reed-Solomon codes are known and the information required to generate them is described in many references. One such reference is Practical Error Correction Design for Engineers, published by Data Systems Technology Corp., Broomfield, Colo. For purposes of this application, it is necessary to know that the Reed-Solomon code generates redundancy terms, herein called P and Q redundancy terms, which terms are used to detect and correct data errors.
In the system shown in FIG. 2, ECC 42 unit is coupled to bus 45. The bus is individually coupled to a plurality of data disk drives, numbered here 47, 48, and 49, as well as to the P and Q term disk drives, numbered 51 and 53 through Small Computer Standard Interfaces ("SCSIs") 54 through 58. The American National Standard for Information Processing ("ANSI") has promulgated a standard for SCSI which is described in ANSI document number X3.130-1986.
Bus 45 is additionally coupled to large output buffer 22. Buffer 22 is in turn coupled to computer 10. In this system, as blocks of data are read from the individual data disk drives, they are individually and sequentially placed on the bus and simultaneously transmitted both to the large buffer and the ECC unit. The P and Q terms from disk drives 51 and 53 are transmitted to ECC 42 only. The transmission of data and the P and Q terms over bus 45 occurs sequentially. The exact bus width can be any arbitrary size with 8-, 16- and 32-bit wide buses being common.
After a large block of data is assembled in the buffer, the calculations necessary to detect and correct data errors, which use the terms received from the P and Q disk drives, are performed within the ECC unit 42. If errors are detected, the transfer of data to the computer is interrupted and the incorrect data is corrected, if possible.
During write operations, after a block of data is assembled in buffer 22, new P and Q terms are generated within ECC unit 42 and written to the P and Q disk drives at the same time that the data in buffer 22 is written to the data disk drives.
Those disk drive systems which utilize known error correction techniques have several shortcomings. In the systems illustrated in FIGS. 1 and 2, data transmission is sequential over a single bus with a relatively slow rate of data transfer. Additionally, as the error correction circuitry must wait until a block of data of predefined size is assembled in the buffer before it can detect and correct errors therein, there is an unavoidable delay while such detection and correction takes place.
As stated, the most common form of data transmission in these systems is serial data transmission. Given that the bus has a fixed width, it takes a fixed and relatively large amount of time to build up data in the buffer for transmission either to the disks or computer. If the large, single buffer fails, all the disk drives coupled thereto become unusable. Therefore, a system which has a plurality of disk drives which can increase the rate of data transfer between the computer and the disk drives and more effectively match the data transfer rate to the computer's maximum efficient operating speed is desirable. The system should also be able to conduct this high rate of data transfer while performing all necessary error detection and correction functions and at the same time provide an acceptable level of performance even when individual disk drives fail.
Another failing of prior art systems is that they do not exploit the full range of data organizations that are possible in a system using a group of disk drive arrays. In other words, a mass storage apparatus made up of a plurality of physical storage devices may be called upon to operate as a logical storage device for two concurrently-running applications having different data storage needs. For example, one application requiring large data transfers (i.e., high bandwidth), and-the other requiring high frequency transfers (i.e., high operation rate). A third application may call upon the apparatus to provide both high bandwidth and high operating rate. Known operating techniques for physical device sets do not provide the capability of dynamically configuring a single set of physical storage devices to provide optimal service in response to such varied needs.
It would therefore be desirable to be able to provide a mass storage apparatus, made up of a plurality of physical storage devices, which could flexibly provide both high bandwidth and high operation rate, as necessary, along with high reliability.